The present invention relates, in general, to a multi-focus ophthalmic lens, and more particularly to a simple, economical method of making such a lens.
The manufacture of prescription eyeglass lenses is a complex art, since it involves huge numbers of possible optical prescriptions. The simplest lens is a spherical surface with a single focal length, but most lenses are more complicated, the typical ophthalmic lens requiring compound curves wherein spherical surfaces are ground simultaneously with sphero-cylindrical surfaces to produce what is known as a torically curved surface. Such surfaces may be further compounded with prismatic power, and may also include multifocal elements, such as bifocal or trifocal segments. The combinations required to meet the needs the large number of people who wear prescription eyeglasses are almost endless.
Lenses having complex compound curves can be produced through a multitude of techniques, using a wide variety of optical materials. For example, one current practice is to inventory a relatively large array of semi-finished lens blanks that already have their front spherical surfaces mass-produced to a prefinished form so that the final prescription can be completed by grinding the necessary compound curves on the rear, or ocular, surface of the lens, which is the surface nearest the eye. Almost every ophthalmic lens produced by a prescription laboratory involves the grinding of a compound toric and prismatic surface on the ocular surface, starting with a partially finished lens blank having a front surface which has been completed through the use of mass production technology. The front surface almost always has a planar or a simple spherical surface, and may also include a multifocal minor lens component affixed onto or formed in the front surface by the producer of the semifinished lens blank.
Numerous other manufacturing techniques are capable of producing the same optical results as are obtained in the manner described above. For example, one method used in the past was that of mass-producing semi-finished lens blanks with concave spherical surfaces formed on the ocular side and optionally including multifocal or minor lens segments, or elements, on that ocular surface. The front surfaces of these lens would then be ground and polished to provide the required prismatic, toric surfacing required for meeting the required prescription of a patient. With some care in calculating the amount and location of the grinding and polishing, the end product was still a lens with the same net compound powers as that of the above-discussed modern practice of rear toric surfacing. However, there is a major complication involved in front surface toric grinding and polishing, for such compound convex surfaces are very difficult to achieve with the degree of accuracy required for optical use. This is due primarily to the fact that it is extremely difficult to provide the wide range of motion required in a grinding and polishing machine to produce accurate convex curvatures. The grinding of a concave toric curve requires much less violent motion of the grinding machine, not only creating less wear and tear on the machinery, but also permitting a wider range of curves to be produced and resulting in more accurate surfaces.
Because of the foregoing problem, the laboratories which produce the large majority of ophthalmic lenses clearly prefer ocular side grinding and polishing over front surface polishing for toric curves. However, when ocular toric surfacing is elected for its inherent simplicity, that forces the location of multifocal minor lens components to front surface of the lens, principally because any grinding of the ocular surface after placement of the multifocal segment changes the thickness, and thus the power, of the segment. The grinding and polishing to prescription could not be done before placement of the minor lens element on the ocular surface because that would require matching the shape of the front surface of the minor lens to all of the possible complex curvatures of the ocular surface, while still retaining the correct net power for the lens. An inventory of hundreds of thousands of individual bifocal buttons would be needed for such matching, which would be totally impractical. Thus, the grinding of toric ocular surfaces, in accordance with current practice in the art, forces the multifocal elements to be located on the front surface of the lens.
The placement of a multifocal minor lens element on the front surface of the lens produces its own problems, however, for such a placement presumes prior knowledge of the thickness of the completed lens after the prescription has been ground into the rear surface. Such prior knowledge of the thickness of the lens behind, and supporting, the multifocal minor lens segment is extremely important, because the overall finished lens thickness determines the true and effective power of the minor lens segment. But in reality the multifocal element must be placed on the surface before it is known what prescription will be ground on the lens blank. Accordingly, the mass producers of semi-finished lens blanks containing front surface multifocal minor lens elements, or segments, can, at best, only make an educated estimate of the actual finished lens thickness, based on the average, or likely, prescription applications of the multifocal blank. They cannot accurately predict the final prescription lens thickness because this thickness depends on not only the prescription required by the specific patient, but also on the effective diameter of the eyeglass frame which is selected by the patient. For example, a specific prescribed lens power might result in a very thin finished lens if that lens were to be placed in a very small diameter eyeglass frame, whereas the same lens power could easily become many times thicker if the lens were designed for insertion into a very large diameter eyeglass frame. Since the power of the multifocal segment is largely dependent on the lens thickness in the region of the segment, the "add" power of the minor lens portion can be significantly affected by the thickness of the major lens behind the minor lens portion and this can result in very large departures from the desired lens power. Multifocal power discrepancies of over 50% can, and often are, found in lenses produced by prescription laboratories because of this effect. This may result in the lens falling outside of acceptable optical tolerances, resulting in rejection of the lens and a consequent financial loss. However, in many cases the discrepancy is dismissed because the lens manufacturer have been unable to alter the basic problem.
The ordinary procedure used by a prescription laboratory in the manufacture of multifocal lenses involves the selection of a semi-finished lens blank carrying a minor lens portion which is marked by the mass producer as having a certain "add" power. The distance prescription parameters required for the lens are then calculated without regard for the add power of the minor lens, and the required prescription is then ground on the ocular surfaces of the lens. However, the grinding away of the lens material behind the minor lens portions varies the power of the minor lens so that upon completion of the prescription grinding, the power of the minor lens portion may be significantly different from the "add" power originally selected. Although it is mathematically possible to recalculate the "add" power once the lens principal thickness has been calculated, this is not ordinarily done due to the sheer complexity of such correction factors. Of course, an optician discovering the "add" power error after producing it, could easily remake another lens using a new lens having a corrected "add" segment, but this would result in a very unacceptable cost and a waste of productivity.
From the foregoing it is seen that when multifocal minor lens segments are placed on the front surface of a main ophthalmic lens, the accuracy of the "add" power provided by the minor lens suffers, but by this placement of the minor lens, ocular side surfacing becomes available, and this is highly desirable. On the other hand, when a multifocal minor lens segment is placed on the ocular side of an ophthalmic lens, the "add" power provided by the minor lens segment remains accurate; however, toric surfacing must then be done on the front surface of the lens and this is more difficult. If the multifocal minor lens segments could be placed on the ocular side of the main lens while still allowing ocular side toric surfacing, "add" power errors would be avoided and toric surfacing would remain simple. But this would be the case only if the multifocal segment was not placed on the ocular surface until after the toric surfacing had been completed. Otherwise, the significant grinding and polishing required for producing a toric surface on a semifinished lens would essentially destroy the multifocal lens segment.
In order to allow placement of a minor lens segment on the ocular surface of a lens after it has been finished, it would be necessary to provide multifocal minor lens segments having front surface curvatures which would match all possible ocular toric surface curvatures so that the minor lens could be adhesively secured to the rear surface of the major lens. Unfortunately, in order to do this an inordinate number of minor lens segments would be required. For example, for a single type of bifocal element, such as a "flat-top 25 mm" bifocal, seventeen different bifocal segments are required to encompass an ordinary power range from plus one diopter to plus five diopter "add" power, in 0.25 diopter steps. However, this assumes that only a single, known curvature was provided on the rear surface of the main lens, so that the front surfaces of these seventeen segments could be finished to that curvature. In reality, however, 320 different spherical curve possibilities are available if the ocular concave surface is varied between plano and minus 20 diopters in increments of 0.0625 diopters, assuming that each curve is ground very precisely. When variations due to finishing tolerances are taken into account, there may be many more curve possibilities. Furthermore, even if tolerance variations are ignored, if each spherical curve is compounded with a cylindrical curve ranging between zero and five diopters (also in 0.0625 diopter steps) then there are eighty possible cylindrical values for each spherical curve, producing 25,600 different curve combinations. In addition, the axis of each cylindrical curve can assume any rotational position from zero to 180 degrees in one degree steps so that the number of curve combinations now increases to 25,600 times 180, or 4,608,000 different toric curves possible on the rear surface of the main lens. Since there are seventeen "add" powers for each type of bifocal, the number of segments required for just one complete inventory of flat top 28 mm bifocal segments would be 78,336,000 in order to closely match the curvatures of the front surfaces of the minor lens components to the rear curvatures of the major lens components. Such matching would be necessary to provide the close fits necessary to achieve accurate optical powers. Curve mismatches could be filled with the optically clear adhesives, but this is unsatisfactory, for such filling causes serious optical power errors. Of course, such an inventory is impractical, if not impossible, for any single retailer or wholesale laboratory to maintain, and for this reason rear surface multifocal minor lens elements are not provided in today's market.
The provision of near vision segments on the rear surface of an ophthalmic lens is highly desirable, since, as explained above, the variations in lens thickness due to differences in prescription and differences in lens diameter, as well as other variations in the lenses, play virtually no role in determining the "add" power provided by a rear-mounted or minor lens element. Thus, if the required inventory for such a lens could be reduced to, for example, seventeen different elements for a given type of minor lens, i.e., one for each of the add powers desired, the manufacture of such multifocal lenses would be very attractive, since a rear surface multifocal element provides superior optics.